igneous
Obsidian
Volcanic Glass (Amorphous silicon dioxide with trace impurities)
Hardness of 5 to 5.5 on the Mohs scale, typically black to dark brown in color, vitreous (glassy) luster, amorphous crystal structure with no true cleavage, conchoidal fracture, and specific gravity of approximately 2.35 to 2.60.
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Physical properties
Hardness of 5 to 5.5 on the Mohs scale, typically black to dark brown in color, vitreous (glassy) luster, amorphous crystal structure with no true cleavage, conchoidal fracture, and specific gravity of approximately 2.35 to 2.60.
Formation & geological history
Forms as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Geological age varies widely depending on the volcanic deposit, ranging from recent Holocene flows to millions of years old.
Uses & applications
Historically used for arrowheads, spear points, and cutting tools due to its extremely sharp conchoidal edges. Modern uses include surgical scalpels, decorative gemstones, and lapidary art.
Geological facts
Because it cools so quickly without forming a crystal lattice, obsidian is technically a mineraloid rather than a true mineral. Freshly broken edges of obsidian can be only a few nanometers thick, making them sharper than the best surgical steel.
Field identification & locations
Easily identified by its dark, glassy appearance and characteristic curved conchoidal fracture surfaces. Commonly found in areas with past volcanic activity, such as the western United States, Mexico, Italy, and Japan.
